1. Field of the Invention
The present invention relates to an optical modulator and an optical-modulator-integrated laser diode, and more particularly, to an optical modulator and an optical-modulator-integrated laser diode suitable for application to high-speed optical communications.
2. Description of the Background Art
In the field of optical communications, an optical modulator having an absorption layer of quantum-well structure has hitherto been known. The absorption layer comprises a well layer involving a comparatively narrow bandgap between the valence band and the conduction band, and a barrier layer which involves a wide bandgap and is formed on either side of the well layer.
FIG. 1 is a band diagram showing an absorption layer of multiple quantum well structure having a plurality of well layers and barrier layers. In FIG. 1, Egb denotes a bandgap of the barrier layer; xcex94Ec denotes a band offset between the quantum level of electrons within the well layer and the conduction band of the barrier layer; and xcex94Ev denotes a band offset between the quantum level of positive holes within the well layer and the valence band of the barrier layer.
The performance of the optical modulator can be evaluated by means of, for example, an extinction ratio. Here, the extinction ratio is the ratio of the maximum intensity Imax to the minimum intensity Imin (Imax/Imin) of light emitted from an optical modulator when light of certain intensity enters the optical modulator. Alternatively, the extinction ratio may be expressed in decibels [10log (Imax/Imin)]. The extinction ratio assumes a greater value as the bandgap of the barrier layer increases. For this reason, the bandgap of the barrier layer in a related-art optical modulator is usually set to 1.05 eV or thereabouts.
The greater the bandgap of the barrier layer, the greater the chirp that is likely to arise in the optical modulator. FIG. 2 is a graph for describing the chirp-related characteristic of the related-art optical modulator. As shown in FIG. 2, the horizontal axis represents a modulator bias voltage applied to the optical modulator, and the vertical axis represents an xcex1 parameter expressing the level of chirp. Three lines shown in FIG. 2 respectively represent the xcex1 parameter obtained as a result of incident light which induces an optical current of 15 mA, the xcex1 parameter obtained as a result of incident light which induces an optical current of 10 mA, and the xcex1 parameter obtained as a result of incident light which induces an optical current of 2 mA.
As shown in FIG. 2, the xcex1 parameter of the optical modulator increases as the amount of resultant optical current increases; that is, as the intensity of light entering the optical modulator increases. More specifically, when the modulator bias voltage is 0V and the optical current is 2 mA, the xcex1 parameter assumes a value smaller than 1. When the optical current is 10 mA, the xcex1 parameter assumes a value of about 2. Further, when the optical current is 15 mA, the xcex1 parameter assumes a value of about 3.
When an optical modulator is used as the light source of an optical communications system, the xcex1 parameter is usually required to assume a value smaller than 1.5. The results shown in FIG. 2 demonstrate that the related-art optical modulator fails to satisfy the performance of the light source of the optical communications system in a range of incident light of high intensity. As mentioned above, the related-art optical modulator involves a problem of inability to simultaneously satisfy demands for incident light of increased intensity and for reduced chirp.
The present invention has been conceived to solve such a drawback and is primarily aimed at providing an optical modulator capable of simultaneously satisfying demand for incident light of increased intensity and demand for reduced chirp.
The present invention is also aimed at providing an optical modulator integrated diode integrally comprising the optical modulator and a laser diode.
The above objects of the present invention are achieved by an optical modulator having an absorption layer of quantum-well structure. The absorption layer includes a well layer having a first bandgap between a valence band and a conduction band. The absorption layer also includes a barrier layer having a second bandgap between a valence band and a conduction band. The second bandgap is set so as to be greater than the first bandgap and be equal to or less than 0.946 eV.
The above objects of the present invention are achieved by an optical modulator integrated laser diode. The integrated laser diode includes the optical modulator described above as well as a laser diode for causing a laser beam of predetermined wavelength to enter an absorption layer of the optical modulator.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.